The present invention relates to a method of controlling a brushless DC motor, and particularly to technology that can be effectively adapted to control the speed of a brushless DC motor without using any particular magnetic sensor for detecting the rotational position.
Owing to its maintenance-free and highly reliable performance, the brushless DC motor has been extensively used in a variety of fields, such as driving the rotary head of a video tape recorder (VTR) and driving the disk in a disk memory device. In the brushless DC motor, a drive current is fed successively into the windings wound on a plurality of magnetic pole pieces of the stator to generate a revolving magnetic field, and the rotor is rotated in synchronism with the revolving magnetic field. In the brushless DC motor, in general, the rotational position of the rotor must be detected to determine the timing of the commutation of the drive electric current, and a magnetic sensor such as a Hall element is added to the motor for this purpose.
In an attempt to decrease the size of the motor and to decrease the consumption of electric power in recent years, there has been proposed technology as disclosed in, for example, Japanese Patent Laid-Open No. 69489/1988, according to which an electronic component such as a magnetic sensor is not attached to the motor, but the rotational position of the rotor is detected by monitoring the drive current of the motor, thereby enabling the size of the motor to be decreased. The above publication discloses a method wherein a high-frequency current is supplied to a plurality of windings of the stator to form short current pulses, and a peak in the amplitude of this current is detected to detect the rotational position of the rotor relative to the stator, thereby to determine the timing of commutation.